1. Technical Field
The present invention relates to a technique of realizing suppression of the variation of optical output and suppression of radiation noise in the transient power control of a discharge lamp that contains a small amount of mercury or no mercury.
2. Related Art
In the case of using a discharge lamp for a lamp of an automobile, the light intensity must be quickly increased after starting the lighting of the discharge lamp. Accordingly, transient power control is performed such that, immediately after the lighting, an electric power larger than the steady lighting state power is supplied to the discharge lamp. Then, the electric power applied to the discharge lamp is reduced gradually with the lapse of time.
In one type of discharge lamp, a small amount of mercury is sealed. In an environmentally friendly type of discharge lamp, there is no mercury (a so-called mercury-free type). In the latter type, transient power control is executed in view of the variance of the lamp voltage at the initial stage of the lighting, the variance of the rising characteristics of light beam at the time of the lighting, etc.
For example, in one related art structure a lamp voltage (or a signal voltage corresponding to the lamp voltage) of the discharge lamp immediately after the lighting is detected and stored as an initial value. Then, a change value of the lamp voltage (voltage difference) with reference to the initial value is calculated and an electric power supplied to the discharge lamp is controlled based on the change value (see Japanese patent publication JP-A-2003-338390).
In the discharge lamp containing mercury, during a time period from the start of the lighting to the steady lighting, since the change value of the lamp voltage is large and the degree of the correlation between the lamp voltage and the optical output is high, there is employed a method in which the lamp voltage is detected to control the electric power supplied to the lamp.
In contrast, in the discharge lamp of mercury-free type, since the change value of the lamp voltage is small during a time period from the start of the lighting to the steady lighting, it is difficult to obtain a correlation between the lamp voltage and the optical output. Thus, it is necessary to use a control method different from the transient power control method. For example, the following method is proposed when a discharge lamp with a rated power of 35 W is used.
(1) A constant electric power of 75 W is applied to the lamp at the time of starting the lighting.
(2) Where the change value of the lamp voltage with reference to the lamp voltage (initial value) just after lighting is represented as “ΔVL”, when ΔVL reaches a threshold value (ΔVL1), the electric power supplied to the lamp is reduced to a value determined according to ΔVL.
(3) When ΔVL further increases to reach another threshold value (ΔVL2), a timer control is started to reduce the electric power supplied to the lamp gradually with the time lapse to converge to 35 W. Incidentally, as the timer control, the electric power supplied to the lamp is reduced gradually with the increase of the voltage at a capacitor by using an integration circuit constituted by the capacitor and a resistor.
FIG. 8 illustrates the temporal changes of the electric power “Pw” supplied to the lamp, the lamp voltage “VL” and the terminal voltage “Vc” of a capacitor for the timer control in the case of starting the lighting of the discharge lamp from a state where a luminous tube thereof is cooled (a so-called cold start). The meanings of time points t1, t2, t3, t4 and a time period Tn are as follows.
t1 represents a time point where ΔVL reaches ΔVL1.
t2 represents a time point where ΔVL reaches ΔVL2.
t3 represents a time point where noise generation starts.
t4 represents a time point where noise generation terminates.
Tn means a time period during which noise is generated (t3 to t4).
In this example, the initial value of the lamp voltage is 25 volts, and the electric power supplied to the lamp is set to 75 W when ΔVL increases with the lapse of time and the time reaches the time point t1. When the time reaches t1, the electric power supplied to the lamp is reduced in accordance with ΔVL. Then, when the time reaches the time point t2, the timer control starts. That is, the charging of the capacitor for the timer control is started and Vc increases gradually. The electric power supplied to the lamp is reduced gradually in a reverse phase relation with the increasing curve of Vc and finally converges to 35 W (in this example, the saturation value of the lamp voltage is 45 volt).
In the noise generation period Tn started from the time point t3 (for example, 10 to 20 seconds after the starting of the lighting), a state of the discharge lamp is unstable and so there arise a problem that electromagnetic noise is radiated during this time period.
In the related art circuit configuration, there is a problem in that it is difficult to obtain good rising characteristics of the optical output and to suppress the influence of the electromagnetic noise.
In the discharge lamp containing mercury, one of actions of mercury is that the temperature increase of the luminous tube is promoted so that light can be emitted even in a cooled state of the luminous tube. That is, in the discharge lamp of mercury-free type, since the action of the mercury is not exerted, it is required to increase the electric power applied to the discharge lamp thereby to increase the temperature of the luminous tube.
To this end, the discharge lamp of mercury-free type is designed so that the luminous tube has a large thickness so as to be durable with an excessive electric power applied thereto.
Thus, the transient power control for the discharge lamp of mercury-free type requires a long time period from the start of the lighting to the stable state of the discharge as compared with the discharge lamp containing mercury. As a result, when the electromagnetic noise generated during this time period is radio noise, the noise may adversely influence various kinds of electronic devices such as a radio or television.
As a method of suppressing the generation of the electromagnetic noise during the noise generation period Tn, it has been experimentally proven that a method of supplying more electric power to the discharge lamp is effective. However, in the case of increasing the electric power applied to the discharge lamp to a degree capable of suppressing the noise, there arises a problem that a large amount of overshoot arises in the rising characteristics of the optical output, or the degradation of the luminous tube is accelerated.
FIG. 9 is a graphical diagram which exemplarily shows the temporal changes of the optical output “L”, the applied electric power “Pw” and the terminal voltage “Vc” in the cold start. This figure shows the time constant of the integration circuit including the capacitor for the timer control.
In this case, the increase of Vc becomes gentle and the noise is suppressed by relatively increasing the electric power applied to the discharge lamp during the time period Tn. However, since the electric power applied to the discharge lamp becomes excessive, an amount of the overshoot (“Ov” exaggeratively shown in the figure) of the optical output becomes large.
In this manner, in the related art technique, there is a problem in that it is difficult to realize both the suppression of the noise and the improvement of the rising characteristics of the optical output or that the control and the circuit configuration for realizing it.